Taxation of Global Income of Residents

yes he can

with REF To case ITA 726/PUNE/205, when a resident who is an ordinarily resident is liable to pay tax on foreign income, going by reverse logic he can set off foreign loss with Indian income.

yes you can but only against income from long term capital gain alone allowed to set off.

Is there any clause in DTAA with USA which says that Income from HP earned outside India by a ordinarily resident is not taxable in India and hence no requirement to disclose the same in Indian Tax Returns?

If yes, please share the relevant section or clause or article which provides so.

Which section provides for setting off such LTCL incurred outside India with the Indian Income?

To my knowledge I never came across such case but one wise thing is to disclose the income in your return. There are two advantages:
1. If not allowed to set off against Indian income the IT Authorities themselves will raise the objection.
2. If you go for appeal and fight it will become a land mark case.